The photorefractive effect involves light-induced charge redistribution in a nonlinear optical material to produce internal electric fields which, by virtue of the optical nonlinearity, produce local changes in the index of refraction such that dynamic, erasable holograms are formed which diffract light. The photorefractive effect is achieved by exposing the material to an optical intensity pattern consisting of bright and dark regions, such as formed by interfering two coherent laser writing beams of the same polarization. Mobile charge generated in the material migrates to form internal space charge electric fields which create refractive index variations due to the electrooptic effect. These variations in refractive index in the photorefractive material are known as index gratings. The index gratings diffract light and are useful for a variety of applications, including storage of holographic images, diffractive optical elements, and photorefractive two-beam coupling.
Inorganic crystals exhibiting the photorefractive effect are well known in the art as described in Gunter and Huignard, "Photorefractive Materials and Their Applications", Vols. I and II ("Topics in Applied Physics", Vols. 61 and 62) (Springer, Berlin, Heidelberg, 1988). Inorganic photorefractive crystals have been fabricated into optical articles for the transmission and control (change in phase, intensity, or direction of propagation) of electromagnetic radiation, as well as for holographic image and data storage.
However, it is technically difficult to fabricate such crystals into desired large area samples or thin-layered devices such as optical wave guides or multiple layer stacks. Further, it is difficult to dope crystalline material with large concentrations of dopants in order to achieve desired property improvements, such as increase in the speed and/or magnitude of the photorefractive effect, because dopants are often excluded from the crystals during growth.
Certain polymeric photorefractive materials have been described by Ducharme et al., U.S. Pat. No. 5,064,264, and Schildkraut et al., U.S. Pat. No. 4,999,809. These polymeric materials can be fabricated into thin-layered devices such as optical wave guides and multilayer stacks. Further, they can be readily doped with materials to improve a photorefractive effect.
Schildkraut describes a photorefractive device having a layer of material comprising a sensitizer, a charge transporting layer, a binder, and an organic molecular dipole, which has been poled in an electric field at elevated temperatures so that the alignment of the molecular dipoles remains for long times at ambient temperatures. Although the material is shown to have light-induced changes in measured properties, Schildkraut does not show the formation of a photorefractive grating to demonstrate a photorefractive device.
Ducharme et al. describe photorefractive materials comprising a polymer, a nonlinear optical chromophore, a charge transport agent, and optionally a charge generator. The charge transport agent can be dispersed in the polymer binder or alternatively covalently bonded to or incorporated in the polymer backbone to form a photoconductive polymer. To achieve superior mechanical, optical, and ease-of-fabrication properties, it is desired to utilize polymers such as polymethacrylate, polystyrene, polycarbonate or the like which are unfortunately nonphotoconductive. The addition of a charge transport agent to suitable nonphotoconductive polymer to provide the requisite photoconductivity can diminish desired properties such a glass transition temperature and also, in particular, the nonlinearity and thereby reduce the diffraction efficiency of the article.
Although Ducharme's materials are useful in certain applications, there still is a desire in the industry for a photorefractive article having a higher diffraction efficiency in combination with good mechanical, optical, and ease-of-fabrication properties.
It is therefore an object of the present invention to provide an improved photorefractive article. Other objects and advantages will be apparent from the following disclosure.